Phosphorescent, heavy metal complexes have been shown to form triplet state excitons upon electron-hole recombination. The phosphors may harness the energy of such triplet excitons and convert them into useful light output, which can often be more efficient then fluorescence-based output. As a result, many heavy metal complexes have been used in phosphorescence-based organic light emitting devices (OLEDs). Among the most popular classes of heavy metal complexes used in phosphorescent OLEDs are those that are cyclometallated with bidentate ligands, such as 2-phenylpyridine. In the absence of oxygen, these complexes are often highly emissive in solution due to the large ligand field induced by the metal-carbon bond, which raises the energy of the non-emissive d-d metal centered transitions above the triplet energy of the cyclometalated ligand. The presence of the heavy metal can serve to increase the intersystem crossing rate through spin-orbit coupling and reduce the forbidden character of emission from the triplet state of the ligand. Most often, these complexes display ligand-centered based phosphorescence (3LC). The structures of the ligands can be varied to enhance emission intensity and color purity.